The present invention relates to a failure test apparatus for a fuel-vapor purging system, which is used for internal combustion engines of vehicles.
There is a known fuel-vapor purging system that causes fuel vapor to be temporarily adsorbed by adsorbents in a canister. Fuel that is adsorbed while a vehicle is running is purged, into the intake system to prevent fuel evaporated in the fuel tank from escaping into the air. In such a fuel-vapor purging system, if there is a hole is in the purge pipe or if the pipe comes off for some reason, fuel leaks and escapes into the air from the canister or the fuel tank. It is therefore necessary to automatically detect whether there is a leak.
An existing system provides a differential pressure between the inside and outside of the fuel-vapor purging system and monitors the behavior of the internal pressure to detect the presence of a leak. For instance, intake vacuum pressure is applied to the fuel-vapor purging system, and then the intake and discharge passages are closed by valves, which makes the fuel-vapor purging system airtight. Then, the internal pressure in the fuel-vapor purging system is measured to determine whether the system is sealed.
The aforementioned intake and discharge passage valves may be the source of a failure. If a valve failure occurs, purging may not be performed properly or fuel vapor may escape into the air from the air-inlet port of the canister. Compared to a hole in the system, a valve failure has a different effect on the internal pressure of the fuel-vapor purging system, and the previously described leak test cannot be used to detect a valve failure.
With regard to a valve failure, conventionally, vacuum pressure is applied to the fuel-vapor purging system through the intake system of an internal combustion engine solely for testing the valves, and the behavior of the internal pressure in this system is checked to detect a valve failure (Japanese Unexamined Patent Publication (KOKAI) No. Hei 5-180101).
However, performing of two kinds of tests, one for holes and one for valve failure, requires that the sequence of applying vacuum pressure to the fuel-vapor purging system, disabling purging, and enabling purging be repeated at least twice. This varies the air-fuel ratio in the intake system over a relatively long period of time. This may result in heavy emissions over a relatively long period of time.
Accordingly, it is an object of the present invention to provide a failure test apparatus for a fuel-vapor purging system that reduces air-fuel ratio variation caused by the testing.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a failure test apparatus for a fuel-vapor purging system is provided. The fuel-vapor purging system adsorbs fuel vapor in a fuel tank into a canister and purges fuel in the canister to an intake system of an internal combustion engine as needed. The system has at least one valve. The apparatus includes first test means and second test means. The first test means provides a differential pressure between the inside and the outside of the fuel-vapor purging system. The first test means measures the internal pressure with the fuel-vapor purging system when in an airtight condition and determines whether a leak exists in the fuel-vapor purging system from the behavior of the internal pressure. The determination by the first test means includes a differential-pressure forming process of creating a differential pressure between the inside and the outside of the fuel-vapor purging system, a sealing process for making the fuel-vapor purging system airtight when the differential pressure exists, and a differential-pressure releasing process for releasing the differential pressure. The second test means measures the internal pressure of the fuel-vapor purging system in association with one or more of the processes to thereby detect a failure in the valve based on the behavior of the internal pressure.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.